1. Field of the Invention:
This invention relates in general to methods for conducting subsurface and marine seismic surveys and in particular to methods particularly suited for conducting three-dimensional subsurface and marine seismic surveys.
2. Description of the Prior Art:
The concept of three-dimensional seismic surveying is well known in the prior art. Heretofor, in order to accurately construct an image of an unknown subsurface structure in three dimensions, without introducing a directional bias to the data, each common reflection point must be sampled from a large number of azimuths and distances or "offsets". This is generally accomplished by taking repetitive measurements and utilizing statistical processing methods to focus the image. The small channel recording systems currently available do not readily permit the sampling necessary to accomplish three-dimensional surveying without the necessity of adding shotpoints, repetitively moving the receivers, and/or repeatedly driving the seismic sources over the recording equipment to obtain the necessary azimuths and offsets. In view of the difficulty in obtaining these measurements several alternative methods have been proposed.
One such alternative method is the so-called "seisloop" method. This method utilizes deploying the availabe seismic receivers in a loop and imparting seismic waves or "shooting" wherever possible around the loop. This technique provides reasonable azimuthal information; however, offset information is generally limited in most areas of the survey.
A second alternative method is referred to as the "swath" technique. This approach utilizes several parallel lines of receivers separated by a distance greater than the in-line receiver interval. A series of seismic waves are imparted into the receiver configuration in an in-line or split spread manner. In this manner, fairly uniform offset information is provided along the center line of the swath; however, the outer line have considerably less uniform offset distributions. Shotpoint movement is limited in this technique to directions parallel to the receiver lines (to avoid driving over receiver lines), therefore a minimal amount of azimuthal information is acquired. This is further complicated by the fact that virtually all of the azimuthal data acquired have very short shot-receiver offsets.
In land surveys the designer of three-dimensional surveys must be concerned with noises commonly called ground roll. These noises are not as common in marine surveys, but do exist and must be considered especially in shallow water. Additionally, noise may be created by scattered reflections from subsea irregularities such as islands or mounds. These are described in a paper by Larner et al. 1983, "Coherent Noise In Marine Seismic Data," Geophysics, Volume 488 pp. 854-886.
As noted above, each of the aforementioned techniques suffer certain defects and it may also be shown that neither of the above-referenced methods can provide the uniform lateral sampling necessary to properly sample coherent noises or the information coupling between the source, receiver and substance to allow adequate processing. Therefore, it should be appararent that a need exists for an improved seismic survey method which can be utilized to obtain three-dimensional subsurface seismic surveys.